Process for increasing the concentration of normal hydrocarbons in a stream

ABSTRACT

A process increases the concentration of normal paraffins in a feed stream comprising separating a naphtha feed stream into a normal paraffin rich stream and a non-normal paraffin rich stream. The non-normal paraffin rich stream is isomerized over a first isomerization catalyst to convert non-normal paraffins to normal paraffins and produce a first isomerization effluent stream. An iso-C4 stream is separated from the first isomerization effluent stream and isomerized over a second isomerization catalyst to convert iso-C4 hydrocarbons to normal C4 hydrocarbons and produce a second isomerization effluent stream. The normal paraffin rich stream, the normal paraffins in the first isomerization effluent stream and/or the second isomerization effluent stream may be fed to a steam cracker.

FIELD

The field is processes for increasing the concentration of normal hydrocarbons in a feed stream and specifically separating out various fractions of a naphtha stream to convert iso-paraffins into normal paraffin in an isomerization zone for producing a feed stream for a steam cracker.

BACKGROUND

Ethylene and propylene are important chemicals for use in the production of other useful materials, such as polyethylene and polypropylene. Polyethylene and polypropylene are two of the most common plastics found in use today and have a wide variety of uses. Uses for ethylene and propylene include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohol.

The great bulk of the ethylene consumed in the production of the plastics and petrochemicals such as polyethylene is produced by the thermal cracking of higher molecular weight hydrocarbons. Steam is usually mixed with the feed stream to the cracking reactor to reduce the hydrocarbon partial pressure and enhance olefin yield and to reduce the formation and deposition of carbonaceous material in the cracking reactors. The process is therefore often referred to a steam cracking or pyrolysis.

The composition of the feed to the steam cracking reactor affects the product distribution. A fundamental basis of this is the propensity a some hydrocarbons to crack more easily than others. The normal ranking of tendency of the hydrocarbons to crack to ethylene is normally given as: normal paraffins; iso-paraffins; olefins; naphthenes, and, aromatics. Benzene and other aromatics are particularly resistant to steam cracking and undesirable as cracking feed stocks, with only the alkyl side chains being cracked to produce the desired product.

The feed to a steam cracking unit is also normally a mixture of hydrocarbons varying both by type of hydrocarbon and carbon number. This variety makes it difficult to separate less desirable feed components, such as naphthenes and aromatics; from the feed stream by fractional distillation. The normal paraffins and the non-normal paraffins can be separated by an adsorption process. Increasing the concentration of normal paraffins in a stream can improve the quality of a feedstock to the steam cracking unit.

Common feeds to steam crackers include light naphtha, which is concentrated in C5-C6 hydrocarbons, and LPG, which comprises C3-C4 hydrocarbons. Light naphtha streams typically contain a mixture of n-paraffins, iso-paraffins, naphthenes and aromatics. It is generally not possible to procure light naphtha streams that are concentrated in n-paraffins. Similarly, LPG streams typically contain a mixture of n-butane, iso-butane, and propane; but streams concentrated in n-butane are not commonly available.

One way to upgrade light naphtha is first to separate the naphtha into a normal paraffin rich stream and a non-normal paraffin rich stream; and subsequently convert a substantial amount of the non-normal paraffin stream in an isomerization zone in the presence of a catalyst into normal paraffins.

An efficient process for separating and converting the iso-paraffins in light naphtha to normal paraffins would significantly increase the profitability of steam cracking operations by increasing the yield of high value ethylene and propylene.

BRIEF SUMMARY

A process increases the concentration of normal paraffins in a feed stream comprising separating a naphtha feed stream into a normal paraffin rich stream and a non-normal paraffin rich stream. The normal paraffin rich stream may be fed to a steam cracker. The non-normal paraffin rich stream is passed over a first isomerization catalyst to convert non-normal paraffins to normal paraffins and produce a first isomerization effluent stream. An iso-C4 stream is separated from the first isomerization effluent stream and isomerized over a second isomerization catalyst to convert iso-C4 hydrocarbons to normal C4 hydrocarbons and produce a second isomerization effluent stream. The second isomerization effluent stream may be fed to a steam cracker.

Alternatively, after separating a naphtha feed stream into a normal paraffin rich stream and a non-normal paraffin rich stream, the process comprises separating the non-normal paraffin rich stream to provide an iso-C6 paraffin rich stream and a methylcyclopentane or C6 cyclic rich stream; and passing the iso-C6 paraffin rich stream over a first isomerization catalyst to convert non-normal paraffins to normal paraffins and produce an isomerization effluent stream.

In a further alternative, after separating a naphtha feed stream into a normal paraffins rich stream and a non-normal paraffins rich stream; isomerizing an iso-C4 paraffin rich stream, which may be derived from other petrochemical complex streams or derived from the non-normal paraffins rich stream, over an isomerization catalyst to convert iso-C4 paraffins to normal C4 paraffins and produce an isomerization effluent stream. A normal C4 paraffin rich stream is separated from the isomerization effluent stream and the normal C4 paraffin rich stream is fed to the step of separating a normal paraffins stream from non-normal paraffins stream and desorbing normal paraffins from an adsorbent.

Additional details and embodiments of the invention will become apparent from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic view of a conversion unit.

DESCRIPTION OF THE INVENTION

The present disclosure endeavors to separate normal paraffins from a light naphtha stream for ideal steam cracker feed. The process employs a normal paraffin-non-normal hydrocarbon separation to extract normal paraffins from the light naphtha stream and transports the normal paraffins to a steam cracking unit. Furthermore, the non-normal hydrocarbons are converted to normal paraffins and transported to a steam cracking unit. The non-normal hydrocarbons, which include iso-paraffins, naphthenes and aromatics, can optionally undergo an additional separation to remove the C6 cyclics and any C7+ components from the C5 and C6 iso-paraffins. The C6 cyclics include methylcyclopentane (MCP), cyclohexane and benzene. The C5 and C6 iso-paraffins can be isomerized to increase the concentration of normal paraffins and then subjected to separation. The C5+ paraffins can be recycled back to the normal-non-normal separation while C4 hydrocarbons can be further separated to remove normal C4 paraffins for feed to the steam cracking unit. Separated iso-C4 paraffins can be reverse isomerized to equilibrium concentrations of normal-C4 paraffins which can be removed and forwarded to the stream cracking unit. Normal C4 hydrocarbons can be used to desorb normal paraffins in the normal-iso paraffin separation. The entire desorbent stream comprising C4-C6 normal paraffins with perhaps some normal C7 paraffins can be fed to the stream cracking unit.

The term “C_(X)” is to be understood to refer to molecules having the number of carbon atoms represented by the subscript “x”. Similarly, the term “C_(X)−” refers to molecules that contain less than or equal to x and preferably x and less carbon atoms. The term “C_(X)+” refers to molecules with more than or equal to x and preferably x and more carbon atoms.

The naphtha feed stream is preferably a hydrotreated light naphtha stream comprising substantially C5 and C6 hydrocarbons having a T90 between about (60° C.) and about 90° C. The end point is taken to minimize the presence of hydrocarbons with more than six carbon atoms in the feed. No more than about 30 wt % C7+ hydrocarbons, preferably no more than about 20 wt % C7+ hydrocarbons and more preferably no more than about 10 wt % C7+ hydrocarbons can be present in the light naphtha feed stream. The naphtha feed stream may comprise normal paraffins, iso-paraffins, naphthenes, and aromatics.

We have found that normal paraffins are more prone to crack to olefins in a steam cracking unit. Hence, it is desired to increase the concentration of normal paraffins in the feed stream 10. The first step in the process is a step of separating the naphtha feed stream into a normal paraffin-rich stream and a non-normal paraffin-rich stream. Normal molecules are defined to mean straight chain molecules such as normal butane, normal hexane, and normal pentane. The most efficient process for such a separation utilizes adsorption. In an aspect, an adsorbent separation unit 12 is used to separate normal paraffins from non-normal paraffins.

As used herein, the term “a component-rich stream” means that the rich stream coming out of a vessel has a greater concentration of the component than the feed to the vessel. As used herein, the term “a component-lean stream” means that the lean stream coming out of a vessel has a smaller concentration of the component than the feed to the vessel.

The naphtha feed stream is delivered to the process in a feed line 10 and passed to the adsorbent separation unit 12. The feed stream in feed line 10 is passed through a valve 101 in the adsorbent separation unit 12 which delivers the feed to an appropriate bed in an adsorbent vessel 46.

The feed stream in feed line 10 is separated into a normal paraffins stream and a non-normal paraffins stream. Straight chain normal paraffins of the naphtha mixture selectively enter or occlude into the porous structure of the adsorbent components but branched or cyclic non-normal chain paraffins do not enter the pores. The non-normal paraffins exit the process as a raffinate stream. In order to provide a useful method for separation of normal from non-normal paraffins, it is necessary to desorb the occluded normal paraffins. In the disclosed process, normal butane will be abundantly available and can be used as a desorbent to desorb normal paraffins in an extract-desorbent stream. If the normal paraffins are to be sent to a steam cracking unit, there is efficiently no need to separate the desorbent normal butane from the extract normal paraffins.

The adsorbent used in the adsorption vessel preferably comprises aluminosilicate molecular sieves having relatively uniform pore diameters of about 5 Angstroms. The preferred adsorbent is provided by commercially available type 5A molecular sieves produced and sold by UOP LLC.

The adsorbent vessel 46 may comprise a series of vertically spaced, separate beds interconnected by a pipe between the bottom of one bed and the top of its upstream adjacent bed. The valve 101 may comprise a manifold arrangement or a rotary valve for advancing the points of inlet and outlet of respective streams in a downstream direction. The adsorbent vessel 46 operates in an upflow mode, although downflow may be suitable. The adsorbent vessel 46 is shown to have four beds I-IV for simplicity, but it may have more beds such as eight, twelve or twenty-four beds.

The feed stream is introduced through feed line 10 through valve 101 which is positioned to send the feed stream through line 17 into the adsorbent bed I. The extract and desorbent is withdrawn from a top of the desorption bed III in line 33, transported through the valve 101 in an extract line 20 to either a separation vessel to separate desorbent from extract or as feed to a steam cracking unit 150. The desorbent is introduced through desorbent line 45 through valve 101 which is positioned to send the desorbent through line 45 into the bottom of the desorbent bed III. The raffinate is withdrawn from a top of the adsorption bed I through raffinate line 23, through valve 101 and to the raffinate fractionation column 24.

Simulated countercurrent flow is achieved by periodically advancing downstream the point of introducing the feed stream and the desorbent while simultaneously and equally advancing downstream the point of withdrawal of raffinate and extract. The adsorbent bed I is defined as the zone bounded between the feed stream inlet and the raffinate outlet; the primary rectification bed II is defined as the zone bounded between the raffinate outlet and the desorbent inlet; the desorption bed III is defined as the zone bounded between the desorbent inlet and the extract outlet; and the secondary rectification bed IV is defined as the zone bounded between the extract outlet and the feed stream inlet. Typical liquid phase operation is preferred, for example, at temperatures of the from about 50° C. to about 300° C., and more particularly no more than about 260° C., and pressures of from slightly superatmospheric to about 30 atmospheres.

Relatively less adsorbed raffinate is withdrawn from the adsorption vessel 46 in the raffinate line 23 through the valve 101 and enters the raffinate fractionation column 24. Since it is desired to obtain a normal paraffin product, the raffinate fractionation column 24 is operated to separate two fractions, a raffinate overhead stream rich in normal paraffin desorbent comprising, in an embodiment, C4− normal paraffins and a bottoms stream rich in non-normal paraffin raffinate comprising C5+ paraffins. The raffinate overhead stream is withdrawn from the raffinate fractionation column 24 in an overhead line 28, condensed in a cooler 29 and fed to a separator 30. A portion of the condensed raffinate overhead is recycled to the raffinate fractionation column 24 as reflux through a reflux line 31 and the remaining portion of the condensed raffinate overhead is withdrawn through a net raffinate overhead line 32. The raffinate overhead stream comprises normal paraffin desorbent which can be recycled to the adsorption vessel 46 in desorbent line 45. Another portion of the raffinate overhead stream in line 48 can be recovered or taken as steam cracker feed and fed to the steam cracker unit 150 in lines 48, 20 and 110.

The term “column” means a distillation column or columns for separating one or more components of different volatilities. Unless otherwise indicated, each column includes a condenser on an overhead of the column to condense and reflux a portion of an overhead stream back to the top of the column and a reboiler at a bottom of the column to vaporize and send a portion of a bottoms stream back to the bottom of the column. Feeds to the columns may be preheated. The top pressure is the pressure of the overhead vapor at the vapor outlet of the column. The bottom temperature is the liquid bottom outlet temperature. Overhead lines and bottoms lines refer to the net lines from the column downstream of any reflux or reboil to the column. Stripper columns may omit a reboiler at a bottom of the column and instead provide heating requirements and separation impetus from a fluidized inert media such as steam. Stripping columns typically feed a top tray and take main product from the bottom.

As used herein, the term “separator” means a vessel which has an inlet and at least an overhead vapor outlet and a bottoms liquid outlet and may also have an aqueous stream outlet from a boot. A flash drum is a type of separator which may be in downstream communication with a separator that may be operated at higher pressure.

The raffinate bottoms stream is withdrawn from raffinate fractionation column 24 through a bottoms line 25 where a portion of the raffinate bottoms flows through a reboiler line 26, reboiler heater 49 and returns heated to the raffinate fractionation column 24. The remaining portion of said raffinate bottoms flows through line 27 as a non-normal paraffin rich stream, particularly rich in non-normal C5 and C6 paraffins. The raffinate fractionation column 24 operates in bottoms temperature range of about 199 to about 221° C. and an overhead pressure of about 276 to about 552 kPa (gauge).

The desorbent displaces the selectively adsorbed normal paraffins from the solid adsorbent in desorbent bed III of adsorbent vessel 46. The desorbent and extract are withdrawn in line 33 through the valve 101. Normally, the desorbent and the extract are separated to recycle the desorbent stream back to the adsorption vessel. However, in an embodiment, sufficient desorbent is produced later in the process as will be explained hereinafter. Moreover, the normal C4 desorbent is an excellent steam cracker feed. So, the normal C5 and C6 paraffin extract desorbed from the adsorbent by the normal C4 paraffin desorbent may all be fed together to the steam cracking unit 150 to produce a higher selectivity to C3 olefins. Hence, in the process 2, an extract-desorbent column may be obviated.

The non-normal paraffin rich stream particularly rich in non-normal C5 and C6 paraffins can be isomerized to increase the concentration of normal C5 and C6 paraffins to equilibrium levels. However, it has been discovered that the conversion to normal paraffins in an isomerization zone can be increased by removing a portion of the C6 cyclic hydrocarbons, such as cyclohexane, methylcyclopentane, and benzene, in the isomerization feed stream passing into the isomerization zone. Specifically, when the concentration of C6 cyclic hydrocarbons in the stream has been reduced, disproportionation reactions occur which lead to increased amounts of valuable C3 paraffins and C4 paraffins, as well as increases in the per pass conversion of the iso-paraffin hydrocarbons in the feed to normal paraffins. The products from the disproportionation reactions undergo isomerization reactions leading to an increase in yields of normal paraffins. Furthermore, additional conversion to C2 to C4 normal paraffins in the non-normal paraffin rich stream is accomplished via hydrocracking reactions.

In an embodiment, the process locates a raffinate splitter column 50 downstream of the adsorbent vessel 46 to separate C6 cyclic hydrocarbons and any C7+ hydrocarbons from isoparaffins in the non-normal paraffin rich stream in line 27. Since the non-normal paraffin rich stream in line 27 does not contain n-hexane with a normal boiling point of 69° C. because it is removed in the adsorption vessel 46, the separation of C6 cyclics from iso-paraffins is simplified. The lightest C6 cyclic hydrocarbon is methylcyclopentane having a normal boiling point of 72° C. whereas iso-C6 paraffins normally boil at 50-64° C. Hence, the proper ordering of separation steps obviates a difficult split between normal hexane and methylcyclopentane that would be capital and operationally intensive and result in a loss of much of the normal hexane, which is a valuable steam cracker feed.

The raffinate splitter column 50 separates C6 cyclic hydrocarbons and C7+ hydrocarbons from the non-normal paraffin stream in line 27. The raffinate splitter overhead stream in the raffinate splitter net overhead line 56 is rich in C5 and C6 iso-paraffins and can be termed as an iso-C6 paraffin rich stream, an iso-C5 paraffin rich stream, a non-normal, non-cyclic paraffin rich stream or an iso-paraffin rich stream. The non-normal paraffin stream is withdrawn in a raffinate splitter overhead stream from the raffinate splitter column 50 in an overhead line 52, through a cooler 53 and into a separator 54. A portion of said raffinate overhead stream is recycled to the raffinate fractionation column 50 as reflux through a reflux line and the remaining portion of the raffinate splitter overhead stream is withdrawn in net raffinate splitter overhead line 56. The raffinate splitter overhead stream is an iso-C6 paraffin rich stream and is also an iso-C5 paraffin rich stream which can be termed a non-normal, non-cyclic paraffin rich stream or an iso-paraffin rich stream. The non-normal, non-cyclic paraffin rich stream may be fed to an isomerization unit 60 in the net splitter overhead line 56 to increase its normal-paraffin concentration. The raffinate splitter column 50 operates in bottoms temperature range of about 124 to about 154° C. and an overhead pressure range of about 0 to about 138 kPa (gauge).

The raffinate splitter bottoms stream is withdrawn from raffinate splitter column 50 through a bottoms line from which a portion of the raffinate splitter bottoms flows through a reboiler line 59, a reboiler heater 57 and returns to the raffinate splitter column 50. The remaining portion of the raffinate splitter bottoms stream flows through a net splitter bottoms line 64 as a cyclic hydrocarbon stream rich in cyclic C6 hydrocarbons and benzene and particularly rich in methyl cyclopentane. The cyclic paraffins stream in the net splitter bottoms line 64 can be taken to a reforming unit to produce aromatic hydrocarbons or sent to the steam cracker.

The non-normal, non-cyclic paraffin rich stream in the net raffinate splitter overhead line 56 may be combined with a hydrogen stream in a first hydrogen line 62 and heated by heat exchange with reactor effluent and fed to higher isomerization unit 60. In the higher isomerization unit 60, the iC5 hydrocarbons and the iC6 hydrocarbons, in the presence of hydrogen provided by hydrogen line 62 and a higher isomerization catalyst, are converted to increase the concentration of normal paraffins: ethane, propane, normal butane, normal pentane and normal hexane. Three reactions promote the production of normal paraffins iso-paraffin disproportionation reactions, reverse isomerization of iso-paraffins, and paraffin hydrocracking reactions.

Cracking of some of the paraffins can occur in the higher isomerization unit 60 to produce C4− paraffins. Moreover, the conversion of iC5 and iC6 paraffins increases significantly via disproportionation reactions due to the fact that the non-normal, non-cyclic paraffin rich stream in the net splitter overhead line 56 is passed into the higher isomerization unit 60 lean in cyclic C6 hydrocarbons. It is believed that the paraffin disproportionation reactions occur by the combination of two iso-paraffin paraffins followed by scission into one lighter hydrocarbon and one heavier hydrocarbon. For example, two iC5 paraffins can combine and form an iC4 paraffin and an iC6 paraffin in the presence of hydrogen. The iC4 paraffins can further react via disproportionation to form a C3 paraffin and an iC5 paraffin. A significant portion of the produced iC4 paraffins also converts to normal C4 paraffins via isomerization reactions in the isomerization zone. Production of C3 and C4 normal paraffins via disproportionation and isomerization reactions occurs with low production of low-value undesired methane as a cracked product. Thus, there is an increase in the overall yield of the normal paraffins in the first isomerization unit 60.

In the higher isomerization unit 60, hydrocracking of the C5 and C6 components occurs to produce methane, ethane, propane, and isobutane. The isobutane can further react via disproportionation reactions and/or isomerization to further produce normal paraffins. For embodiments that include a lower isomerization unit 120, the isobutane produced in the higher isomerization unit 60 can be separated and fed to the lower isomerization unit to convert the isobutane to normal paraffins. In this respect, the isobutane that is produced in the higher isomerization unit 60 is considered a desired product in addition to the C2 to C6 normal paraffin products.

The higher isomerization catalyst in the higher isomerization unit 60 may include chlorided alumina, sulfated zirconia, tungstated zirconia or zeolite-containing isomerization catalysts. The higher isomerization catalyst may be amorphous, e.g., based upon amorphous alumina, or zeolitic. A zeolitic catalyst would still normally contain an amorphous binder. The catalyst may comprise a sulfated zirconia and platinum as described in U.S. Pat. No. 5,036,035 and European patent application 0 666 109 A1 or a platinum group metal on chlorided alumina as described in U.S. Pat. Nos. 5,705,730 and 6,214,764. Another suitable catalyst is described in U.S. Pat. No. 5,922,639. U.S. Pat. No. 6,818,589 discloses a catalyst comprising a tungstated support of an oxide or hydroxide of a Group IVB (IUPAC 4) metal, preferably zirconium oxide or hydroxide, at least a first component which is a lanthanide element and/or yttrium component, and at least a second component being a platinum-group metal component. These documents are incorporated herein for their teaching as to catalyst compositions, isomerization operating conditions and techniques. An advantage of a non-chlorided catalyst, such as a sulfated zirconia catalyst, is the absence of chloride omitting further treatment of the effluent streams from the isomerization unit 60. If chlorided alumina catalyst is used as the isomerization catalyst, a chloriding agent will be added to the higher isomerization feed stream 61.

The higher isomerization process conditions in the higher isomerization unit 60 include an average reactor temperature usually ranging from about 40° to about 250° C. Reactor operating pressures generally range from about 100 kPa to 10 MPa absolute. Liquid hourly space velocities (LHSV) range from about 0.2 to about 25 volumes of hydrocarbon feed per hour per volume of catalyst. Hydrogen is admixed with or remains with the higher isomerization feed to the higher isomerization unit to provide a mole ratio of hydrogen to hydrocarbon feed of from about 0.01 to 20. The hydrogen may be supplied totally from outside the process or supplemented by hydrogen recycled to the feed after separation from higher isomerization reactor effluent.

Contacting within the higher isomerization unit 60 may be effected using the higher isomerization catalyst in a fixed-bed system, a moving-bed system, a fluidized-bed system, or in a batch-type operation. The reactants may be contacted with the bed of higher isomerization catalyst particles in upward, downward, or radial-flow fashion. The reactants may be in the liquid phase, a mixed liquid-vapor phase, or a vapor phase when contacted with the higher catalyst particles, with a mixed phase or vapor phase being preferred. The higher isomerization unit 60 may be in a single reactor 66 or in two or more separate higher isomerization reactors 67, 68, and 69 with suitable means therebetween to ensure that the desired isomerization temperature is maintained at the entrance to each zone.

The reactions in the higher isomerization unit 60 generate an exotherm across the reactors so the higher isomerization effluent streams need to cooled between reactors. For example, a first higher isomerate stream from a first isomerization reactor 67 may be heat exchanged with the higher isomerization feed stream in the higher isomerization feed line 61 comprising the non-normal, non-cyclic paraffin rich stream mixed with hydrogen to cool the higher isomerate and heat the higher isomerization feed stream. Moreover, a second higher isomerate stream from a second higher isomerization reactor 68 may be heat exchanged with the higher isomerization feed stream comprising the non-normal, non-cyclic paraffin rich stream mixed with hydrogen upstream of the heat exchange with the first higher isomerate steam to cool the higher isomerate stream and heat the higher isomerization feed stream. Additionally, a third isomerate stream from the third isomerization reactor 69 may be heat exchanged with the higher isomerization feed stream comprising non-normal, non-cyclic paraffin rich stream mixed with hydrogen upstream of the heat exchange with the second higher isomerate stream to cool the higher isomerate and heat the higher isomerization feed stream. Since hydrocracking reactions are very exothermic, two to five higher isomerization reactors in sequence enable improved control of individual reactor temperatures and partial catalyst replacement without a process shutdown. A first, higher isomerization effluent stream comprising an increased concentration of normal paraffins exits the last higher isomerization reactor 69 in the higher isomerization unit 60 in a higher isomerization effluent line 65.

A debutanizer column 70 separates a first isomerization effluent stream into a debutanizer overhead stream comprising C4− paraffins and a debutanized bottoms stream comprising C5+ paraffins. The debutanizer overhead stream is withdrawn from the debutanizer column 70 in a debutanizer overhead line 72 and condensed in a cooler and passed into a separator 74. A portion of the condensed debutanizer overhead stream is recycled to the debutanizer column 70 as reflux through a reflux line and the remaining portion of the condensed debutanizer overhead stream is withdrawn in line 76 and fed to a deethanizer splitter column 80. A debutanizer off gas stream is taken from the separator overhead in line 73. The debutanizer off gas in the net overhead line 73 may be scrubbed (not shown) to remove chlorine if a chloride isomerization catalyst is in the isomerization unit 60 and passed to fuel gas processing or sent to a steam cracking unit 150 for further recovery of hydrogen and ethane that can be used as steam cracking feed.

The debutanized bottoms stream is withdrawn from the debutanizer column 70 through a bottoms line from which a portion of the debutanized bottoms stream flows through a reboiler line 77, a reboiler heater and returns to the debutanizer column 70. The remaining portion of the debutanized bottoms flows through a net debutanized bottoms line 79 rich in normal and iso C5-C7 paraffins, is cooled by heat exchange with the first isomerization effluent stream in the first isomerization effluent line 65 and is recycled to the feed line 10 to the adsorption separation unit 12 for separation of the normal paraffins from the non-normal paraffins. The debutanizer column 70 operates in bottoms temperature range of about 153 to about 188° C. and an overhead pressure range of about 1.3 to about 1.5 MPa.

The deethanizer column 80 separates the condensed debutanizer overhead stream in the line 76 into a deethanizer overhead stream rich in C2− hydrocarbons and a deethanized bottoms stream rich in C3 and C4 paraffins. The deethanizer overhead stream is withdrawn from the deethanizer column 80 in a deethanizer overhead line 82 and condensed in a cooler and passed into a separator 84. The condensed deethanizer overhead stream is recycled to the deethanizer column 80 as reflux through a reflux line and the uncondensed off gas is removed in a deethanizer net overhead line 83. The deethanizer off gas in the net overhead line 83 may be scrubbed to remove chlorine if a chloride isomerization catalyst is in the higher isomerization unit 60 and passed to fuel gas processing. The debutanizer off gas stream in the debutanizer overhead in line 73 may also be processed with the deethanizer off gas stream in the deethanizer net overhead line 83. The deethanizer and debutanizer off gas may both be subjected to contact with sponge oil taken from the debutanized bottoms stream to absorb heavy paraffins entrained in the off gas streams to preserve them for use more valuable than fuel. The deethanizer column 80 operates in bottoms temperature range of about 100 to about 130° C. and an overhead pressure range of about 210 to about 300 kPa (gauge).

The deethanized bottoms stream is withdrawn from the deethanizer column 80 through a bottoms line from which a portion of the deethanized bottoms stream flows through a reboiler line 87, a reboiler heater and returns to the deethanizer column 80. The remaining portion of the deethanized bottoms flows through a net deethanized bottoms line 89 comprising C3 and C4 paraffins and is fed to a depropanizer column 90.

The depropanizer column 90 separates the deethanized bottoms stream into a depropanizer overhead stream comprising propane and a depropanized bottoms stream comprising C4 paraffins. The depropanizer overhead stream is withdrawn from the depropanizer column 90 in a depropanizer overhead line 92 and fully condensed in a cooler and passed into a separator 94. A portion of the condensed depropanizer overhead stream is recycled to the depropanizer column 90 as reflux through a reflux line and the remaining condensed depropanizer overhead stream is taken as a C3 stream in a depropanizer product line 95 which may be further upgraded such as by steam cracking or dehydrogenation to make an olefinic C3 stream.

The depropanized bottoms stream is withdrawn from the depropanizer column 90 through a bottoms line from which a portion of the depropanized bottoms stream flows through a reboiler line 97, a reboiler heater and returns to the depropanizer column 90. The remaining portion of the depropanized bottoms stream comprising C4 paraffins flows through a net depropanized bottoms line 99. Thus, a C4 paraffin rich stream is separated from the first, higher isomerization effluent stream in the first, higher isomerization effluent line 65.

The C4 paraffin rich stream has a large concentration of normal C4 paraffins which make an excellent steam cracker feed. So, separation of the normal C4 paraffins from the non-normal C4-paraffins may be performed to produce steam cracker feed.

In an embodiment, the C4 paraffin rich stream is fed to a deisobutanizer column 100 to separate the C4 stream into an iso-C4 paraffin rich overhead stream and a normal C4 paraffin rich bottoms stream. The deisobutanizer overhead stream rich in isobutane is withdrawn from the deisobutanizer column 100 in a deisobutanizer overhead line 102 and fully condensed in a cooler and passed into a separator 104. A portion of the condensed deisobutanizer overhead stream is recycled to the deisobutanizer column 100 as reflux through a reflux line and the remaining condensed deisobutanizer overhead stream is taken as an isobutane rich stream in a deisobutanizer net overhead line 105. The isobutane stream may be fed to a butane isomerization unit 120 to increase the concentration of normal butane paraffins in the isobutane stream in the deisobutanizer net overhead line 105.

The deisobutanized bottoms stream is withdrawn from the deisobutanizer column 100 through a bottoms line from which a portion of the deisobutanized bottoms stream flows through a reboiler line 107, a reboiler heater and returns to the deisobutanizer column 100. The remaining portion of the deisobutanized bottoms flows through a net deisobutanized bottoms line 109 which is rich in normal butane. Thus, a normal butane rich stream is separated from the first, higher isomerization effluent stream in the first higher isomerization effluent line 65. The normal butane rich stream is an excellent steam cracker feed, so a portion of the normal butane rich, deisobutanized bottoms stream may be fed in a steam cracker feed line 110 to the steam cracking unit 150. The other portion of the normal butane rich, deisobutanized bottoms stream in a replenish desorbent line 112 may be used to replenish desorbent in the desorbent line 45. The deisobutanizer column 100 operates in bottoms temperature range of about 60 to about 80° C. and an overhead pressure range of about 517 to about 758 kPa (gauge).

The isobutane rich stream in the deisobutanizer net overhead line 105 may be combined with a hydrogen stream in a hydrogen line 113 and optionally a fresh isobutane stream in fresh isobutane line 114 to provide a second, butane isomerization feed stream in butane isomerization feed line 116. The butane isomerization feed stream is heated by heat exchange with a butene isomerization effluent stream and isomerized in a second, butane isomerization unit 120. In the butane isomerization unit 120, the isobutane paraffins, in the presence of hydrogen provided by hydrogen line 113 and a butane isomerization catalyst, are converted into normal butane to attain equilibrium levels of normal butane.

In addition to isobutane-normal butane isomerization, the conversion of isobutane via disproportionation reactions can also occur. The iC4 hydrocarbons can react via disproportionation to form a C3 hydrocarbon and an iso-C5 paraffin. The iso-C5 paraffins can also isomerize to equilibrium forming normal pentane. Thus, there is an increase in the overall yield of the normal paraffins to propane, normal butane and normal pentane in the butane isomerization unit 120.

The butane isomerization catalyst may include chlorided alumina, sulfated zirconia, tungstated zirconia or zeolite-containing isomerization catalysts. The butane isomerization catalyst may be amorphous, e.g., based upon amorphous alumina, or zeolitic. A zeolitic catalyst would still normally contain an amorphous binder. The catalyst may comprise a sulfated zirconia and platinum as described in U.S. Pat. No. 5,036,035 and European patent application 0 666 109 A1 or a platinum group metal on chlorided alumina as described in U.S. Pat. Nos. 5,705,730 and 6,214,764. Another suitable catalyst is described in U.S. Pat. No. 5,922,639. U.S. Pat. No. 6,818,589 discloses a catalyst comprising a tungstated support of an oxide or hydroxide of a Group IVB (IUPAC 4) metal, preferably zirconium oxide or hydroxide, at least a first component which is a lanthanide element and/or yttrium component, and at least a second component being a platinum-group metal component. These documents are incorporated herein for their teaching as to catalyst compositions, isomerization operating conditions and techniques. An advantage of a non-chlorided catalyst, such as a sulfated zirconia catalyst because it does not contain chloride omitting further treatment of the effluent streams from the second, butane isomerization unit 120. If chlorided alumina catalyst is used as the isomerization catalyst, a chloriding agent will be added to the second, butane isomerization feed stream 116.

The butane isomerization conditions in the butane isomerization unit 120 include reactor temperatures ranging from about 40° C. to about 250° C., preferably at reactor temperatures ranging from 90° C. to 204° C. Reactor operating pressures generally range from about 100 kPa to 10 MPa absolute. LHSV range from about 0.2 to about 25 volumes of hydrocarbon feed per hour per volume of catalyst. Hydrogen is admixed with or remains with the butane isomerization feed to the butane isomerization unit to provide a mole ratio of hydrogen to hydrocarbon feed of from about 0.01 to 20. The hydrogen may be supplied totally from outside the process or supplemented by hydrogen recycled to the feed after separation from butane isomerization reactor effluent.

Contacting within the isomerization unit 120 may be effected using the catalyst in a fixed-bed system, a moving-bed system, a fluidized-bed system, or in a batch-type operation. The reactants may be contacted with the bed of catalyst particles in upward, downward, or radial-flow fashion. The reactants may be in the liquid phase, a mixed liquid-vapor phase, or a vapor phase when contacted with the catalyst particles, with a mixed phase or vapor phase being preferred. The butane isomerization unit 120 may be in a single reactor 122 or two or more separate reactors 122 and 124 with suitable means therebetween to ensure that the desired isomerization temperature is maintained at the entrance to each zone. Since the main reaction in the butene isomerization unit is isomerization of isoparaffins to normal paraffins which is endothermic, the temperatures across the reactors decline. Consequently, the butene isomerization effluent needs to be reheated before going to the downstream reactor. For example, a first butane isomerate stream from a first butane isomerization reactor 122 may be heated by heat exchange and fed to a second butane isomerization reactor 124. Moreover, a second butane isomerate stream from the second butane isomerization reactor 124 may be heat exchanged with the butane isomerization feed stream comprising an isobutane-rich stream mixed with hydrogen to cool the second butane isomerate and heat the butane isomerization feed stream. Two or more reactors in sequence enables improved isomerization through control of individual reactor temperatures and partial catalyst replacement without a process shutdown. A second, butane isomerization effluent stream comprising increased normal paraffins exits the last reactor in the butane isomerization unit 120 in a second, butane isomerization effluent line 126.

A stabilizer column 130 separates the butane isomerization effluent stream into a stabilizer overhead stream comprising C3− hydrocarbons and a stabilized bottoms stream comprising C4+ paraffins. The debutanizer overhead stream is withdrawn from the stabilizer column 130 in a stabilizer overhead line 132 and condensed in a cooler and passed into a separator 134. A condensed stabilizer overhead stream is recycled to the stabilizer column 130 as reflux through a reflux line and the uncondensed gases are taken as a butane isomerization off gas in a stabilizer net overhead line 136. The stabilizer off gas in the stabilizer net overhead line 136 may be scrubbed to remove chlorine if a chloride isomerization catalyst is in the butane isomerization unit 120 and passed to fuel gas or other processing.

The stabilized bottoms stream comprising C4+ paraffins is withdrawn from the stabilizer column 130 through a bottoms line from which a portion of the stabilized bottoms stream flows through a reboiler line 137 and a reboiler heater and returns to the deisobutanizer column 130. The remaining portion of the stabilized bottoms flows through a net stabilized bottoms line 139 comprising a mixed butanes stream comprising normal butane and isobutane. The mixed butanes stream is fed back to the deisobutanizer column 100 to separate isobutanes from normal butanes. Thus, the second, butane isomerization effluent stream is fed to the step of separating the C4 stream into an isobutane stream and a normal butane stream. The stabilizer column 130 operates in bottoms temperature range of about 110 to about 130° C. and an overhead pressure range of about 2.1 to about 2.5 MPa.

SPECIFIC EMBODIMENTS

While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims.

A first embodiment of the invention is a process for increasing the concentration of normal paraffins in a feed stream comprising separating a naphtha feed stream into a normal paraffin rich stream and a non-normal paraffin rich stream; isomerizing the non-normal paraffin rich stream over a first isomerization catalyst to convert non-normal paraffins to normal paraffins and produce a first isomerization effluent stream; separating an iso-C4 stream from the first isomerization effluent stream; isomerizing the iso-C4 stream over a second isomerization catalyst to convert iso-C4 hydrocarbons to normal C4 hydrocarbons and produce a second isomerization effluent stream; and feeding the second isomerization effluent stream to a steam cracker. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising separating a C4 stream from the first isomerization effluent stream and separating the C4 stream into the iso-C4 stream and a normal C4 stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising feeding the normal C4 stream to the steam cracker. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising feeding the normal C4 stream to the step of separating a normal paraffins stream from non-normal paraffins stream and desorbing normal paraffins from an adsorbent. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising feeding the normal-C4 stream and the desorbed normal paraffins to a steam cracker together. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising feeding the second isomerization effluent stream to the step of separating the C4 stream into an iso-C4 stream and a normal C4 stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising separating the first isomerization effluent stream to produce a C3 stream and further converting the C3 stream into an olefinic stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising separating a C5 and C6 paraffins stream from the first isomerization effluent stream and recycling the C5 and C6 paraffins stream to the step of separating the naphtha feed stream into the normal paraffin rich stream and the non-normal paraffin rich stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the step of separating the naphtha feed stream into the normal paraffins stream from the non-normal paraffins stream comprises extracting the normal paraffins by use of an adsorbent. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising separating cyclic hydrocarbons from the non-normal paraffin stream before isomerizing the non-normal paraffins stream.

A second embodiment of the invention is a process for increasing the concentration of normal paraffins in a feed stream comprising separating a naphtha feed stream into a normal paraffin rich stream and a non-normal paraffin rich stream; separating the non-normal paraffin rich stream to provide an iso-C6 paraffin rich stream and C6 cyclics rich stream; and isomerizing the iso-C6 paraffin rich stream over a first isomerization catalyst to convert non-normal paraffins to normal paraffins and produce an isomerization effluent stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising separating a C4 paraffin rich stream from the first isomerization effluent stream and separating the C4 paraffin rich stream into the iso-C4 paraffin rich stream and a normal C4 paraffin rich stream; separating an iso-C4 paraffin rich stream from the first isomerization effluent stream; isomerizing the iso-C4 paraffin stream over a second isomerization catalyst to convert iso-C4 paraffins to normal C4 paraffins and produce a second isomerization effluent stream; and feeding the second isomerization effluent stream to a steam cracker. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising separating a C5 and C6 paraffin stream from the first isomerization effluent stream and recycling the C5 and C6 paraffin stream to the step of separating the naphtha feed stream into the normal paraffin rich stream and the non-normal paraffin rich stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising feeding the normal paraffin rich stream to a steam cracker. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising separating a C4 paraffin rich stream from the first isomerization effluent stream and separating the C4 paraffin rich stream into the iso-C4 paraffin rich stream and a normal C4 paraffin rich stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising feeding the normal C4 paraffin rich stream to the steam cracker. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising feeding the normal C4 paraffin rich stream to the step of separating the naphtha feed stream into the normal paraffin stream and the non-normal paraffin stream and desorbing normal paraffins from an adsorbent and feeding the normal-C4 paraffin rich stream and the desorbed normal paraffin stream to a steam cracker together. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising separating the first isomerization effluent stream into a C3 paraffin rich stream and further converting the C3 stream into an olefinic stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the step of separating the normal paraffins stream from the non-normal paraffins stream comprises extracting the normal paraffins by use of an adsorbent.

A third embodiment of the invention is a process for increasing the concentration of normal hydrocarbons in a feed stream comprising separating a naphtha feed stream into a normal paraffins stream and a non-normal paraffins stream; isomerizing an iso-C4 paraffin rich stream over an isomerization catalyst to convert iso-C4 paraffins to normal C4 paraffins and produce an isomerization effluent stream; separating a normal C4 paraffin rich stream from the isomerization effluent stream; and feeding the normal C4 paraffin rich stream to the step of separating a normal paraffins stream from non-normal paraffins stream and desorbing normal paraffins from an adsorbent.

Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated. 

1. A process for increasing the concentration of normal paraffins in a feed stream comprising: separating a naphtha feed stream into a normal paraffin rich stream and a non-normal paraffin rich stream; isomerizing the non-normal paraffin rich stream over a first isomerization catalyst to convert non-normal paraffins to normal paraffins and produce a first isomerization effluent stream; separating an iso-C4 stream from said first isomerization effluent stream; isomerizing said iso-C4 stream over a second isomerization catalyst to convert iso-C4 hydrocarbons to normal C4 hydrocarbons and produce a second isomerization effluent stream; and feeding said second isomerization effluent stream to a steam cracker.
 2. The process of claim 1 further comprising separating a C4 stream from said first isomerization effluent stream and separating said C4 stream into said iso-C4 stream and a normal C4 stream.
 3. The process of claim 2 further comprising feeding said normal C4 stream to said steam cracker.
 4. The process of claim 2 further comprising feeding said normal C4 stream to the step of separating a normal paraffins stream from non-normal paraffins stream and desorbing normal paraffins from an adsorbent.
 5. The process of claim 2 further comprising feeding said normal-C4 stream and said desorbed normal paraffins to a steam cracker together.
 6. The process of claim 2 further comprising feeding said second isomerization effluent stream to the step of separating said C4 stream into an iso-C4 stream and a normal C4 stream.
 7. The process of claim 1 further comprising: separating said first isomerization effluent stream to produce a C3 stream and further converting said C3 stream into an olefinic stream.
 8. The process of claim 1 further comprising separating a C5 and C6 paraffins stream from said first isomerization effluent stream and recycling the C5 and C6 paraffins stream to the step of separating the naphtha feed stream into the normal paraffin rich stream and the non-normal paraffin rich stream.
 9. The process of claim 1 wherein the step of separating the naphtha feed stream into the normal paraffins stream from the non-normal paraffins stream comprises extracting said normal paraffins by use of an adsorbent.
 10. The process of claim 1 further comprising separating cyclic hydrocarbons from said non-normal paraffin stream before isomerizing the non-normal paraffins stream.
 11. A process for increasing the concentration of normal paraffins in a feed stream comprising: separating a naphtha feed stream into a normal paraffin rich stream and a non-normal paraffin rich stream; separating the non-normal paraffin rich stream to provide an iso-C6 paraffin rich stream and C6 cyclics rich stream; and isomerizing the iso-C6 paraffin rich stream over a first isomerization catalyst to convert non-normal paraffins to normal paraffins and produce an isomerization effluent stream.
 12. The process of claim 11 further comprising separating a C4 paraffin rich stream from said first isomerization effluent stream and separating said C4 paraffin rich stream into said iso-C4 paraffin rich stream and a normal C4 paraffin rich stream; separating an iso-C4 paraffin rich stream from said first isomerization effluent stream; isomerizing said iso-C4 paraffin stream over a second isomerization catalyst to convert iso-C4 paraffins to normal C4 paraffins and produce a second isomerization effluent stream; and feeding said second isomerization effluent stream to a steam cracker.
 13. The process of claim 11 further comprising separating a C5 and C6 paraffin stream from said first isomerization effluent stream and recycling the C5 and C6 paraffin stream to the step of separating the naphtha feed stream into the normal paraffin rich stream and the non-normal paraffin rich stream.
 14. The process of claim 11 further comprising feeding said normal paraffin rich stream to a steam cracker.
 15. The process of claim 12 further comprising separating a C4 paraffin rich stream from said first isomerization effluent stream and separating said C4 paraffin rich stream into said iso-C4 paraffin rich stream and a normal C4 paraffin rich stream.
 16. The process of claim 15 further comprising feeding said normal C4 paraffin rich stream to said steam cracker.
 17. The process of claim 12 further comprising feeding said normal C4 paraffin rich stream to the step of separating the naphtha feed stream into the normal paraffin stream and the non-normal paraffin stream and desorbing normal paraffins from an adsorbent and feeding said normal-C4 paraffin rich stream and said desorbed normal paraffin stream to a steam cracker together.
 18. The process of claim 12 further comprising: separating said first isomerization effluent stream into a C3 paraffin rich stream and further converting said C3 stream into an olefinic stream.
 19. The process of claim 12 wherein the step of separating the normal paraffins stream from the non-normal paraffins stream comprises extracting said normal paraffins by use of an adsorbent.
 20. A process for increasing the concentration of normal hydrocarbons in a feed stream comprising: separating a naphtha feed stream into a normal paraffins stream and a non-normal paraffins stream; isomerizing an iso-C4 paraffin rich stream over an isomerization catalyst to convert iso-C4 paraffins to normal C4 paraffins and produce an isomerization effluent stream; separating a normal C4 paraffin rich stream from said isomerization effluent stream; and feeding said normal C4 paraffin rich stream to the step of separating a normal paraffins stream from non-normal paraffins stream and desorbing normal paraffins from an adsorbent. 